This invention relates to circuitry for inserting a system blanking interval into a video signal obtained, for example, from an image pickup device.
The video signals from the image pickup tubes used in a television system include objectionable spurious signals and noise which occur during the retrace and transition from active to inactive portions of the scanning beam travel. Since the retrace portion of the video signal is used to convey the synchronizing components for each television line, it is necessary to provide a baseline during the retrace interval free from such spurious signals and noise in order to insert properly shaped synchronizing components for developing the composite video waveform.
Known circuits for removing the spurious signals occurring during the retrace interval generally comprise a video input signal clamp for establishing black level reference followed by a transistor switch which is operated by a blanking signal so as to literally obliterate the video waveform during the retrace interval. These known circuits, however, often do not produce a properly shaped blanking interval as specified by the television waveform standards promulgated by various television standards committees, such as the U.S. Federal Communications Commission (FCC). Generally, the characteristics of such blanking interval switches in the form of uncontrolled risetimes, interelectrode capacity and variation of storage time with temperature, for example, severely affect the risetime and duration of the rigidly defined blanking interval period in the video output waveform.